1. Field of the Invention
This invention relates to a projection lens for use in a projection apparatus for magnifying and projecting an image on a liquid crystal display screen onto a screen remote from the projection apparatus, and, more particularly, to a telecentric projection lens for use in a projection apparatus which has a color mixing optical element such as a cross prism or a cross mirror assembly for mixing three primary colors disposed between a liquid crystal display screen and the projection lens system.
2. Description of Related Art
Optical lens systens of this type are demanded not only to be compact in overall size but to have high telecentric optical performance, high resolving power, lateral chromatic aberrations and distortion well corrected to provide high quality of photographic images and a wide angle of view. Specifically, because the visibility of liquid crystal display screen is dependent on viewing angles, in order to realize the uniformity in the overall contrast of an image, the projection lens system has to be telecentric. If a telecentric projection lens system is not used in the projection device, a problem of color shading is caused due to a change in optical characteristics of a dichroic prism as the angle at which light rays incident upon the surface of the dichroic prism is varied.
In recent years, there is a tendency for liquid crystal display devices to increases the number of picture elements such as from 640.times.480 picture elements (VGA) and 800.times.600 picture elements (SVGA) to 1240.times.768 picture elements (XGA) and 1280.times.1024 picture elements (SXGA), which arises a strong demand for projection lens systems having high resolving power and providing high quality of images. Further, in compliance with great wishes to see pictures on a large screen at a short distance in a small room, there is a demand for projection lens systems with a more shorter focal length.
In general, projection devices project a magnified image on a screen placed at a higher level than a place on which the projection device is arranged. For this reason, the projection lens system is usually positioned off upward from the horizontal center axis of the liquid crystal display screen and, consequently, has the necessity of having an angle of view sufficient to cover an area 1.2 to 1.6 times as large as the entire area of liquid crystal display screen.
While it is essential for a compact telecentric projection lens system to include a high power convex lens element on a side of the liquid crystal display screen, the projection lens system must has a back focal distance sufficiently long to install a color mixing optical element in a position in front of the liquid crystal display screen, which requires the projection lens system to include a foremost concave lens like that of a retro-focus type of lens. Such a projection lens system is hard to be provided with a wide angle of view and high optical performance due to the difficulty with corrections of aberrations such as field curvature, astigmatism, distortion and lateral chromatic aberrations. Further, due to a change in aberrations during focusing or projecting distance, it is hard for the projection lens system to have desirable resolving power and provide high quality of projected images over a wide range of projecting distances.
One of projection lens systems for use as a compact telecentric system in projection devices of the type having a liquid crystal display device is described in Japanese Unexamined Patent Publication No. 5-157966. This telecentric projection lens system, however, has the difficulty in being installed to a liquid crystal display device having a large number of picture elements because of its small angle of view and great residual aberrations and further encounters the difficult problem of no sufficient space for a color mixing optical element because of a short back focal distance. One of projection lens systems for use in projection devices of the type having color mixing optical element to be disposed between the projection lens system and a liquid crystal display device is described in Japanese Unexamined Patent Publication No. 5-241071. This projection lens system has a long overall length and a large diameter of foremost lens element, which is always undesirable for the compactness of projection lens system. Further, the projection lens system causes a great change in aberrations due to a change in projecting distance and/or image size and, consequently, encounters the difficult problem in obtaining desirable resolving power over a range of projecting distances from the closest to the longest distance.